Examining Threats to Monarch Butterfly Migration

The monarch butterfly may be the most recognized butterfly in the world. With the exception of the Polar Regions, the medium-size butterflies can be found on every continent on Earth. Their spectacular migration in eastern North America, from breeding locations in Canada and the United States to overwintering sites in Mexico, is nothing short of a miracle and has been the subject of decades of study.

Monarchs have four life stages; egg, larva (caterpillar), pupa, and adult (butterfly). The search for milkweed, the only food that monarch larvae eat, is the sole reason for the annual monarch migration.

This is the time of year when monarchs are here, laying their eggs upon the leaves of milkweed plants growing in meadows and fields and along roadsides and rights-of-way. The eggs hatch in 4 or 5 days. Newly-hatched caterpillars grow through 4 larval instars, each time shedding a skin that they’ve outgrown. Before shedding their skin for the fifth and final time, they fasten themselves to twigs and leaves by a sticky, silk-like thread. The green pupae that emerge hang there for 10-15 days, during which time the pupal casings harden, forming the chrysalises from which the adult monarch butterflies emerge.

With the coming of cool fall temperatures and shorter days, the last generation of adult monarchs born here and across much of the northern United States and southern Canada will set out for overwintering sites in stands of Oyamel fir growing high in the mountain forests of central Mexico; 2000 miles away. The entire eastern population will arrive there in late November; so many of them that the combined flapping of their wings creates a clear and constant sound. There they wait for the coming of spring, when they again take flight; this time headed for the Gulf coast states to breed, lay their eggs, and die.

The brood hatched in the southern states will migrate northward, breeding along the way. They arrive here, and at other locations at the northern extreme of their range, beginning in early summer and the cycle starts again.

Milkweed is abundant in much of the United States and Canada. But, as the name indicates, it’s considered by most to be a weed and, as such, mowed down or eliminated with chemical herbicides. It’s also being eradicated inadvertently as fields and meadows are replaced with housing, parking lots, shopping malls, and industrial centers. Such efficient suppression of milkweed has long been considered the greatest threat to the survival of eastern North American monarchs.

Anurag Agrawal disagrees. The Cornell University professor of ecology and evolutionary biology (James A. Perkins Professor of Environmental Studies) and senior author of a paper titled “Linking the Continental Migratory Cycle of the Monarch Butterfly to Understand Its Population Decline,” published April 4, 2016 in the journal Oikos, believes “lack of milkweed … is unlikely to be driving the monarch’s population decline, as the problem appears to occur after they take flight in the fall.” Milkweed is not a food source for adult butterflies during their southern migration in autumn.

Agrawal and the scientist/co-authors of the paper: lead author Hidetoshi Inamine, a graduate student in the field of ecology and evolutionary biology at the time; Stephen P. Ellner, Horace White Professor of Ecology and Evolutionary Biology; and James P. Springer of the North American Butterfly Association hypothesize that the real threats to monarch populations are a scarcity of autumnal nectar sources, weather, and habitat fragmentation. Their research was funded by Cornell’s Atkinson Center for a Sustainable Future.

While at Cornell Cooperative Extension, I became aware of ongoing public concern about the risk that Bt corn might present to monarchs. Bt corn is bioengineered with a gene from the soil bacterium Bacillus thuringiensis, which is toxic to European corn borer caterpillars, a significant crop pest, but shown to have no effect on many ‘non-target’ organisms (e.g. honeybees, ladybugs).

Bt corn is, however, toxic to monarch butterfly larvae. But monarchs don’t eat corn. The danger is from exposure to Bt corn pollen, which contains crystalline endotoxin from the bacterium genes. When the pollen is dispersed by the wind, it lands on other plants, including milkweed.

A collaborative research effort across the U.S. corn belt and Ontario (University of Guelph), coordinated by the U.S. Department of Agriculture’s Agricultural Research Service, specifically addressed the impact of Bt pollen on monarch butterfly and other caterpillar species. The studies showed that monarch caterpillars have to be exposed to pollen levels greater than 1,000 grains/cm2 to show toxic effects. Caterpillars were found to be present on milkweed during the one to two weeks that pollen is shed by corn, but corn pollen levels on milkweed leaves were found to average only about 170 grains/cm2, posing only a negligible threat.

Photos, from above: Monarch butterfly, and Monarch caterpillar courtesy Cornell College of Agriculture and Life Sciences (CALS).

14 Responses

Your description of the monarch life cycle is pretty accurate (there are 5 larval instars), but don’t accept Agrawal et al.’s argument without skepticism. There is solid evidence published by Pleasants, Oberhauser, and others that the decline in midwestern milkweed is linked directly to a decline in monarch reproduction. And the evidence cited by Agrawal, Inamine, and Davis for the lack of a summer decline is from surveys outside the central monarch range. Drought-induced decline in nectar sources during fall migration is an additional factor, but the decline in monarch abundance in the midwest seems primary. One forecast is that New York may become a more important breeding ground as midwestern reproduction declines.

Hi, Tom. A lot of studies followed the publication of Losey et al., and I think it’s fair to say that Bt corn imposes minimal risk for monarchs. The real threats to monarchs are the decline in breeding habitat in the midwestern U.S., logging and degradation of the overwintering forests in Mexico, and the effects of drought (climate change) and habitat loss through the migratory corridor (loss of nectar, etc.). For the Adirondacks, as you said we have a good amount of milkweed, and you’re right that roadsides should not be mowed from late July to mid September.

As I’ve published in local Adirondack newspapers, there is no dearth of milkweed over much of the Adirondacks, and, although it might be good to encourage its continued abundance in the wild, planting more is probably unnecessary. On the other hand, destroying milkweeds in mid- to late summer, by mowing or other means, will also destroy any Monarch larvae or pupae on them. I’m pleased to say that the public works folks in the Town of Webb, NY, (Old Forge), forego mowing roadside or other wild milkweed stands until after all the adult Monarchs have emerged and gone south.

“Milkweed….. it’s considered by most to be a weed and, as such, mowed down or eliminated with chemical herbicides…

>Ignorance abounds in this society…is why all the senseless damage against the dame nature.

“It’s also being eradicated inadvertently as fields and meadows are replaced with housing, parking lots, shopping malls, and industrial centers….”

> Tell this to those town officials in Clifton Park whose lust for new tax havens are responsible for loss of tons of habitat due to new subdivisions, new Adirondack Tire’s, new car washes, new steel and brick and mortar….

Every plant in an ecosystem has a use… The only exception are invasive species.
Most people have no clue most of these chemicals they use to “purify” their lawns and fields stem from chemicals that were used in war to destroy. Sad. Even something like Dandelions… They have so much use – yet some people are under the impression only grass should be allowed to grow.

“the public works folks in the Town of Webb, NY, (Old Forge), forego mowing roadside or other wild milkweed stands until after all the adult Monarchs have emerged and gone south.”

Very nice and thoughtful. Does this have to do with that organization who went around a few years ago and asked the town officials to ‘hold back’ on their mowing until after the butterfly season was done? Are they still around? They don’t advertise on this site anymore. I would have donated by now if I remembered who they were. Anybody?

“The real threats to monarchs are the decline in breeding habitat in the midwestern U.S., logging and degradation of the overwintering forests in Mexico, and the effects of drought (climate change) and habitat loss through the migratory corridor (loss of nectar, etc.)”

With the new EDA (Environmental Destruction Agency) we can expect more of the same. And what’s with this ‘climate change’ thing. I thought this was all a hoax!

“Their spectacular migration in eastern North America, from breeding locations in Canada and the United States to overwintering sites in Mexico, is nothing short of a miracle”

Indeed it is a miracle (key word) – just like everything else in nature and just like our own existence. Miracles given to us by our Creator God. That’s reason enough to take care of it… Our own selves and the lives of people and plants and animals around us.

Yesterday at the Union Square Farmers’ Market in New York City, where I work once a week, a beautiful big Monarch appeared, a divine vision, perhaps the first on the long journey South. To see one in the City is encouraging. I have a garden in Jackson Heights, as well as a place in Columbia County and our old family camp in the Adirondacks, and I’ve been planting milkweed seeds in all of those places. Perhaps if people could be made more aware of the importance of milkweed for the Monarchs, they would be encouraged to plant it rather than mow it down as a “weed”. Milkweed is quite beautiful in bloom, and has a lovely scent.